In general, a axial fan comprises a casing having a cavity portion in the center, an impeller having a plurality of vanes which rotates together with a rotation axis, a motor for rotating the rotation axis, a motor base for holding the motor. The impeller, the motor and the motor base are housed in the cavity portion of the casing.
A boss is formed integrally with the motor base. A bearing housing of a hollow cylindrical shape provided with a bearing for supporting the rotation axis in the center is fitted and mounted to the boss. A stator of the motor is mounted on the outside of the bearing part, and the rotation axis is mounted inside thereof, rotatably through a bearing. In addition, the motor base is connected to the casing by a plurality of spokes.
Then, when the motor is driven, the rotation axis rotates together with the impeller. By the rotation of the impeller, fluid (air) is sucked into the impeller through one end side of the cavity portion of the casing, or a suction port, and passes through the inside of the casing, and is blown out to the outside of the casing through the other end side of the cavity portion of the casing, or a discharge port. At this time, when it is required to increase the pressure of the fluid blown out to the outside of the casing, a fixed vane is provided in the vicinity of the discharge port of the casing in some case, and the spoke serves as the fixed vane in another case. In addition, the casing, the motor base, and the spoke are formed by molding integrally using a resin or a metal, and the like.
Meanwhile, the axial fan as described above is used to cool an electronic device, by mounting the axial fan to the electronic device and efficiently discharging the heat generated from an electronic component inside the electronic device to the outside of the electronic device. Therefore, high air volume of the axial fan (high blowing out air volume per unit time of the axial fan) is required. In addition, with regard to electronic devices such as a server and the like, the interior space in which the air flows grows smaller and smaller, due to the high density packaging inside the housing. Incidentally, high static pressure (high power of the axial fan for blowing out the air) is required in addition to the above mentioned high air volume to the axial fan for cooling the inside of the housing.
In order to obtain the high air volume and the high static pressure of the axial fan, it is necessary to rotate the motor for rotating the impeller at a high speed. However, when the motor is rotated at a high speed, vibration caused by the high speed rotation of the motor is transmitted to the casing through a bearing, and the vibration is transmitted to the electronic device equipped with the axial fan, there generates a problem that the vibration may occur to the electronic device. In particular, when the vibration caused by the rotation of the motor resonates with the natural frequency of the casing, the vibration grows larger, and, as a result, there may generate a big problem that an abnormal vibration occurs in the electronic device.
With regard to such problems, a axial fan so as to suppress the generation of the vibration, by strengthening the structure of the housing through forming a plurality of reinforcement ribs to the motor base is proposed, as described in Japanese Laid-Open Patent Application Publication No. 2006-57631, for example.